Unable to update field machine name

Problem/Motivation

When saving/updating index fields, if you want to change the machine_name value of the field you will get a "field does not exist" error.

This is especially necessary when adding a field with a vague name (like an aggregated_field) or an entity reference field since it can be nondescript by default ("name" for a referenced taxonomy term).

Steps to reproduce

1. Create or update the machine_name for a field.
2. Save the index fields form

Name is updated

Field is removed and field does not exist error is thrown.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes